A pump jack has an upper and lower shackle which act as friction ratchets to grip the pole tightly as the work platform is being raised and while in use. To lower the work platform, the lower shackle is released and the shaft of the upper shackle which directly grips the pole is rotated to lower the pump jack and work platform. It is very important that the shaft of the upper shackle is positively locked against rotation until the platform is to be lowered. Locking of the shaft should be easily done and its locked condition should be easily verified by the user.
One design presently in production depends upon the crank handle of the upper shackle shaft being jammed behind the pole for locking. This potentially can allow the platform to drop some distance before solid lock-up will occur. Another design requires that the user hold the shaft crank handle against axial spring force while cranking in order to disengage the lock on the shaft. Subsequent locking depends on the user returning the shaft crank handle to a specific position. Both of these designs are awkward to use or have the risk of not locking securely.
The present invention is a crank mechanism that overcomes these faults. It will lock securely and easily, and when unlocked will allow for easy cranking operation.